Electroacoustic transducer having improved transducing element supporting means

ABSTRACT

An electromechanical transducer element is supported within a transducer housing by at least one folded spring-like member having a plurality of generally annular &#39;&#39;&#39;&#39;leaves&#39;&#39;&#39;&#39; each having an annular width relatively large in comparison to its thickness. In operation, air resistance between the leaves or convolutions of the spring-like member serves to acoustically damp undesirable transducer resonances, thereby providing a transducer frequency response characteristic that is relatively uniform throughout the band of interest.

.XFi iifililelZk UlllI/Cu Datum 1 aecu'e 1 1' Bryant 1 May 1, 1973 [54]ELECTROACOUSTIC TRANSDUCER HAVING IMPROVED TRANSDUCING ELEMENTSUPPORTING MEANS [75] Inventor: Herbert William Bryant, Middle town,N.J.

[73] Assignee: Bell Telephone Laboratories lncorp0rated,Murray Hill, NJ.

[22] Filed: Oct. 18, 1971 [21] Appl. No.: 190,207

[52] U.S. C1 ..3l0/9.4, 310/82 [51] Int. Cl .1101! 7/00, H041 17/00 [58]Field of Search ..310/3, 8.2, 8.3, 310/9.19.4', 91/505, 506; 417/222[56] References Cited UNITED STATES PATENTS 3,619,672 11/1971 Nagata..310/9.4

2,488,781 11/1949 Reeves ..3l0/9.2 X 2,453,435 [1948 Haustad ..3l0/9.3 X

3,622,816 11/1971 McGrew ..3l0/9.4

Primary Examiner.l. D. Miller Assistant Examiner-Mark O. BuddAttorney-R. J. Guenther et a1.

7 [5 7] ABSTRACT An electromechanical transducer element is supportedwithin a transducer housing by at least one folded spring-like memberhaving a plurality of generally annular leaves each having an annularwidth relatively large in comparison to its thickness. In operation, airresistance between the leaves or convolutions of the spring-like memberserves to acoustically damp undesirable transducer resonances, therebyproviding a transducer frequency response characteristic that isrelatively uniform throughout the band of interest.

11 Claims, 5 Drawing Figures BACKGROUND OF THE INVENTION 1 Field of theInvention The present invention relates generally to electroacoustictransducers utilized to convert sound energy into electrical energy, andvice-versa, and, more particularly, to such transducers having improvedmeans for supporting an electromechanical transducing element within thetransducer housing.

2. Description of the Prior Art While electroacoustic transducers of thepiezoelectric ceramic type possess advantages of reduced size andincreased stability when compared to other instruments serving the samefunction, various design problems have delayed their widespread adoptionfor use in telephone and other electronic systems. Notable among theseproblems has been the need for a suitable means for supporting thetransducing element within the transducer housing that not only providesa system stiffness which yields a desired frequency responsecharacteristic, but also meets design requirements in terms of stabilitywith age and temperature variations. Conventional transducing elementsupports have included a pair of gaskets, at least one of which isrubber, for supporting opposite faces of the element. Typically, thesegaskets, as for example rings, are compressed against the periphery ofthe faces, so that a pressure seal is provided, and the element held inthe desired position. Unfortunately, while such supporting means arerelatively simple and inexpensive to produce, several disadvantages areinherent in their use. First, and most importantly, the stiffness of thevibratory system is closely tied to the amount of compression applied tothe rings. As a result, changes in the dimensions of the parts, assemblytechniques, or in rubber composition produce varying amounts ofcompression, which in turn, may often adversely affect the transducerfrequency response characteristics, or, at least cause unwantedvariations in such characteristics between individual transducers of thesame lot. Second, the use of a'rubber material in the element supportingstructure could introduce an undesirable and virtually uncontrollabletime dependent variation in the transducer frequency responsecharacteristics, by virtue of changes that occur in the physicalproperties of the rubber as it ages. Additionally, the rubber stiffnessis also quite prone to variations brought about by changes in ambienttemperature, again possibly adversely affecting the overall performanceof the instrument.

Accordingly, it is the broad object of the present invention to providean electroacoustic transducer,

preferably of the piezoelectric ceramic type, having an tively immune tovariations in ambient temperature and relatively stable over longperiods of time.

SUMMARY or THE INVENTION Each of the foregoing and additional objectsare achieved in accordance with the principles of the invention by theprovision, in an electroacoustic transducer, of an improved means forsupporting the transducing element within a housing which includes atleast one folded spring-like member having a plurality of generallyannular leaves each of which has an annular width relatively large inrelation to its thickness. Briefly described, the aforesaid member maybe fabricated by folding a thin strip of moistureproof paper or metalfoil back upon itself a number of times, and by then forming a centralhole in the folded member and appropriately trimming the outer edgesthereof.

The advantageous provision of the supporting means described greatlyfacilitates the attainment of a desired transducer frequency responsecharacteristic, since the stiffness of the transducing element supportand the degree of damping provided thereby are easily controllableduring design. For a given housing, theformer is simply a function ofthe mechanical properties of the material employed, the thickness of thestrip and the number of leaves utilized, while the latter is adjustedsimply by changing the size of the central hole,'and the number ofleaves, thereby producing a corresponding variation in air resistancebetween adjacent leaves as the supporting means vibrates. Additionally,by virtue of the fact that, in accordance with the invention, thesupporting means are constructed from a metal, moisture-proof paper, orother similar material, and do not require the use of a rubber material,excellent stability in transducer performance can be achieved despitechanges in ambient overextended periods of time.

BRIEF DESCRIPTION OF THE DRAWING The aforementioned and other featuresand advantages of the instant invention will become more readilyapparent to persons skilled in the art by reference to the followingdetailed description, when read in light of the accompanying drawing, inwhich:

FIG. 1 is a central cross-sectional view of an electroacoustictransducer constructed in accordance with the principles of theinvention;

FIG. 2 is a fragmentary cross-sectional view of a piezoelectric ceramictransducing element for use in the transducer of FIG. 1;

FIG. 3 is a top view of the means used to support the transducingelement in the transducer of FIG. 1;

FIG. 4 is a sectional front view of the supporting means of FIG. 3; and

FIG. 5 is a graph of a typical frequency response characteristic of atransducer constructed in accordance with the instant invention.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown in centralcross-sectional view an electroacoustic transducer constructed inaccordance with the principles of the invention. The transducercomprises a housing, designated temperature, and

generally at 10, defining an internal chamber 11, and a planarelectromechanical transducing element 12 within the chamber. Means forsupporting the element 12 within the chamber 11, and for spacing theelement y from the top and bottom walls 24 and 23 thereof, in-

clude first and second spring-like members 13 and 14, respectively.

Transducing element 12 may be of the piezoelectric ceramic type, and mayinclude a pair of oppositely poled discs of a polarizable ferroelectricceramic material such as barium titanate, lead zirconate-lead titanate,or sodium potassium niobate, bonded together to form a compositebilaminate element, as shown greatly enlarged in FIG. 2. The individualdiscs may be fabricated in various ways, one of which is fully describedin a concurrently filed patent application, Ser. No. 190,209, filed Oct.18, 1971, by T. C. Austin and H. W. Bryant, entitled the same as theinstant application and assigned to the same assignee. 7

After fabrication, electrodes are affixed to both faces of each ceramicdisc, such as electrodes 12c and 12d on disc 12a and electrodes 122 and12f on disc 12b, shown .in FIG. 2 greatly exaggerated in thickness forthe purposes of illustration. The electroded discs are then placed inalignment, and securely bonded together so that inner electrodes 12d and12e are in electrical contact. A bonding technique found suitable forthe purposes of the instant invention is described in the aforesaidcopending application, but other well-known methods may be equallysatisfactory.

The overall electrical response characteristics of the transducer ofFIG. I depend upon both the acoustics of the housing 10 and the meansutilized to support the transducing element 12 within the chamber 11formed thereby. In certain transducer applications, it is particularlydesirable to achieve a relatively flat or uniform characteristic curveover the frequency band of interest, although it is to be understoodthat other characteristics can also be achieved in accordance with theinvention. If required, a low-frequency rolloff of the desired curve isrelatively easy to obtain, in the case of a piezoelectric ceramictransducer, when it is realized that the ceramic transducing elementrepresents a capacitance at low frequencies. Accordingly, byappropriately choosing the value of resistance connected across thetransducer, as, for example, the input resistance of a preamplifierstage following. the transducer, the corresponding R-C corner frequencymay be suitably located. The high frequency end of the transducercharacteristic typically contains an undesirably high peak located atthe first natural resonant frequency of the mounted transducing element12, as well as higher frequency overtones, all of which must besuppressed or filtered out in order to obtain a flat response. Thissuppression is provided, in part, by the means used to support theelement in accordance with the invention, to be more fully discussedhereinafter, and also by the proper engineering of the acoustics ofhousing 10.

Referring again to FIG. 1, housing 10 includes a cupshaped body member15 and a disc-like frontal plate 16, both of which may be fabricatedfrom a plastic material possessing suitable qualities of heat andmoisture resistance. Frontal plate 16 has centrally drilled therethrougha port or center hole, such as port 17, which is covered by an acousticdamping member 18 either fastened across the hole atop the plate 16 ornested and fastened within a centrally bored recess 19 provided in plate16, as shown in FIG. 1. Port 17 serves as a low-pass filter, and whensuitably dimensional, selectively suppresses the high frequencyovertones noted above. The acoustic resistance or damping member 18,which may conveniently be fabricated from porous sintered steel, helpsto reduce the primary resonance and overtones to an acceptable level. Inassembled form, frontal plate 16 and body member 15 are encased by athin metal shell 20 having a generally U-shaped cross section. A conedmetal spring washer 21 may be used to supply the pressure necessary toensure positive contact between plate 16 and member 15 and to hold theunit in proper alignment.

As stated above, the proper design of means to support transducingelement 12 within chamber 11 of housing 10 is extremely critical to theattainment of the desired transducer frequency response characteristic.Functionally, the means chosen, in order to be effective, must provide afirst natural resonance of the mounted element at a desired frequency,and in addition provide a semirigid clamping action around the elementperiphery that serves to minimize planar or piston-like motion of theelement while inducing or facilitating a cupping or spherical bendingaction in the element, the latter being the only source of electricaloutput. Additionally, the supporting means must be capable of providinga desired degree of mechanical damping, if required, to further reducethe resonant peak.

In accordance with the principles of the instant invention, means .forsupporting element 12 within chamber 11 of housing 10 that meet theabove requirements include first and second folded or accordian pleatedspring-like members 13 and 14, respectively, as depicted in FIG. 1. Aswill be discussed hereinafter, a single such member may, in certaincircumstances, be advantageous.

Each spring-like member'13 and 14, such as member 13, shown greatlyenlarged in top and sectional front views in FIGS. 3 and 4,respectively, maybe fabricated from a thin strip of moisture-proofpaper, metal foil, or other suitable material, folded back upon itself aplurality of times to form a plurality of accordian-like interconnectedleaves, such as leaves 30, 31 and 32. A central hole 35, preferablyround, is next punched or otherwise formed centrally through the member,and the outer periphery trimmed in the general shape'of a circle 36having a diameter 37 slightly larger than the distance 38 between thefolds. As shown in FIG. 1, the outer diameter 37 of the members 13 .and14 is arranged to be slightly greater than the outer diameter oftransducing element 12. Each leaf of members 13 and 14 thus consists ofa generally annular area having an annular width relatively large inrelation to its thickness, said width being sufficiently great toprovide a measurable amount of acoustic damping to the transducingelement support system, due to energy dissipation in the air resistancebetween adjacent leaves during vibration thereof.

During assembly,member 13 is seated upon the inner face of cup-shapedbody member 15, which forms the bottom wall 23 of chamber 11. One end ofa first strip 25 of electrically conductive material is next attached toa terminal rivet 28 extending through the bottom portion of cup-shapedhousing member 15, and the other end positioned atop a portion of thetop leaf 30 of member 13. Transducing element 12 is next seated onmember 13 so that electrode 12f makes electrical contact with strip 25.Cement may be used to fasten element 12 to member 13, thereby preventingmisalignment due to mechanical shock. A second strip 27 of gold platedcopper foil or other conducting material similar to strip 25 is thenplaced in electrical contact with a portion of the outer periphery ofelectrode 12c, that strip having been previously extended along the sidewall 22 and bottom wall 23 of chamber 11 to an electrical connectionwith a second terminal rivet 26 similar to terminal rivet 28. Member 14is next seated on the upper face of transducing element 12. To completeassembly, frontal plate 16 is then aligned with body member 15, andspring washer 21 is placed thereupon. Finally, the entire transducer isclamped together by appropriately bending shell 20, as shown in FIG. 1.Electrical connection between the transducer and an external load orsource is accomplished by appropriately connecting leads to the externalportions of terminals 26 and 28. In the event that member 13 isfabricated from an electrically conductive material, electrical contactbetween electrode 12f and rivet 28 can be made directly through member13, thereby obviating the need for strip 25. However, in thisconfiguration, electrical contact between member 13 and the portion ofstrip 27 passing thereunder must be prevented by the provision of asmall insulating member 29 of a suitable insulating material, as shownin FIG. 1. v

The stiffness of the spring-like members 13 and 14 which supporttransducing element 12 is easily controlled during design byappropriately selecting the mechanical properties of the particularmaterial employed, the thickness of the material, and the number ofleaves utilized. The provision of a desired stiffness in turn controlsthe natural resonance frequencies of the entire support system. For thenon-resonant response given in FIG. 5, the following combination of theabove properties has been found satisfactory, in the case of supportingmembers constructed from metal foil:

Material: 61 ST Aluminum Thickness ofleaves 30, 31, 32: .0016 inchYoungs modulus: l0 lbs./in.

Number of leaves per member: In addition, changes in the degree ofdamping provided by the transducing element supporting means areeffectuated by appropriately controlling the ratio of the annular widthof members 13 and 14 to the thickness of each leaf by changing the sizeof hole 35, adjusting the ratio R between the combined thicknesses ofthe transducing element and fully compressed members 13 and 14 and thedepth of transducer chamber 11. R can be varied by changing the numberof leaves utilized and the dimensions of the transducer housing. Thefollowing dimensions and ratios have been found suitable in thetransducer of FIG. 1:

Outer Diameter 37: .675 inches Distance 38 between folds: .615 inchesDiameter of hole 35: .440 inches Ratio of annular width to thickness: 73

Space factor (1R): 31%

It is to be understood, of course, that other combinations of theaforementioned properties of members 13 and 141, depending upon theparticular transducer characteristics desired, are intended to be withinthe scope of the invention.

From the above description of the transducer of FIG.

. 1, it can be seen that means to support transducing element 12 withinchamber 11 have been provided in accordance with the invention which arequite stable and not prone to change due to the passing of time andambient temperature variations. Since transducing element 12 issemirigidly supported between members 13 and 14, it is buffered againstmechanical damage normally possible with rigid clamping. In addition,the supporting means are easily tailored to meet desired responserequirements, as evidenced by FIG. 5, which depicts the frequencyresponse characteristic of a transducer constructed in accordance withthe invention. As shown, the response is flat within i 1.5 db, in theband from 0.2 to 9.0 kHz.

While a pair of spring-like members 13 and 14 are utilized to supporttransducing element 12 in the instrument of FIG. 1, it is to be notedthat a single such member may on occasion be desirable, as, for example,with a metal-ceramic transducer element mounted on a greatly stiffersupport member than previously described, but, having the same physicalshape as members 13 and 14. In this configuration, element 12 is simplysecured to the uppermost leaf 30 of member 13, by means of cement, epoxyor any other suitable fastening arrangement, and appropriate clearanceis maintained between the upper face of element 12 and the top 24 ofchamber 11.

It is to be noted that a metal-ceramic transducing element, as describedin the above-mentioned copending application, Ser. No. 190,209, filedOct. 18, 1971, by T. C. Austin and H. W. Bryant, may be used in lieu ofthe bilaminar transducing element 12 of FIGS. 1 and 2, in certaincircumstances, proper account being taken of the required element sizesand geometry. For example, in the transducer of FIG. 1, such atransducing element having a ceramic disc whose outer diameter issmaller than the diameter of hole 35 may be employed, or alternatively,a larger ceramic may be utilized in a transducer having a singlesupporting member 13.

In order to more fully appreciate the degree of com pactness of atransducer of the type depicted in FIG. 1, the following typicaldimensions are given by way of illustration only, the instant inventionnot being limited to the sizes stated:

Outside diameter of complete transducer 0.822 inches Thickness ofcomplete transducer 0.302 inches Diameter of transducing element 12 0.59inches Thickness of transducing element 12 0.012 inches element is of anelectromechanical transducer type other than a piezoelectric ceramic.Still further, while the shape of the transducing element has heretoforebeen described as round, or disc-like, it should be clearly understoodthat appropriate modifications to the peripheral shape of the transducerand/or its internal components may sometimes be required.

What is claimed is: l An electroacoustic transducer comprising anelectromechanical transducing element, a housing defining an internalchamber having a top wall and a bottom wall for containing said element,

and means for supporting said element within said chamber, saidsupporting means including at least one spring-like member for spacingsaid element from one of said walls, said member having a plurality ofannular leaves each having an annular width relatively large in relationto its thickness, said leaves being interconnected by hinge portions onthe periphery thereof.

2. The invention defined in claim 1 wherein said electromechanicaltransducing element includes a polarizable ferroelectric ceramicmaterial.

3. The invention defined in claim 2 wherein said ceramic material isselected from the group consisting of barium titanate, leadtitanate-lead zirconate, and sodium potassium niobate.

4. The invention defined in claim 3 wherein said spring-like member is ametal foil.

5. The invention defined in claim 1 wherein said sup' porting meansincludes two spring-like members for spacing said element from said topwall and from said bottom wall.

6. The invention defined inv claim 5 wherein said electromechanicaltransducing element includes a polarizable .ferroelectric ceramicmaterial.

7. The invention defined in claim 6 wherein said ceramic material isselected from the group consisting of barium titanate, leadtitanate-lead zirconate, and sodium potassium niobate.

8. The invention defined in claim 7 wherein said spring-like member is ametal foil.

9. An electroacoustic transducer comprising an electromechanicaltransducing element,

a housing defining an internal chamber having a top wall and a bottomwall for containing said element, and

at least one annular spring-like member for spacing said element fromone of said walls, said member comprising a plurality of interconnectedgenerally circular leaves each having a thickness and an annular widthdetermined by the size of a central hole formed therein, said widthbeing large in relation to said thickness.

10. In an electroacoustic transducer including a transducing element anda housing for containing said element, means for supporting said elementin 'said housing comprising at least one annular spring-like memberhaving a plurality of interconnected leaves each having an annular widthrelatively large in relation to its thickness.

1 1. An electroacoustic transducer comprising an electromechanicaltransducing element,

a housing defining an internal chamber having a top wall and a bottomwall for containing said element and means for supporting said elementwithin sald chamber, said supporting means including at least oneaccordion pleated member comprising a plurality of interconnectedgenerally annular leaves each having an annular width sufficiently greatto provide a measurable amount of acoustic damping.

1. An electroacoustic transducer comprising an electromechanicaltransducing element, a housing defining an internal chamber having a topwall and a bottom wall for containing said element, and means forsupporting said element within said chamber, said supporting meansincluding at least one spring-like member for spacing said element fromone of said walls, said member having a plurality of annular leaves eachhaving an annular width relatively large in relation to its thickness,said leaves being interconnected by hinge portions on the peripherythereof.
 2. The invention defined in claim 1 wherein saidelectromechanical transducing element includes a polarizableferroelectric ceramic material.
 3. The invention defined in claim 2wherein said ceramic material is selected from the group consisting ofbarium titanate, lead titanate-lead zirconate, and sodium potassiumniobate.
 4. The invention defined in claim 3 wherein said spring-likemember is a metal foil.
 5. The invention defined in claim 1 wherein saidsupporting means includes two spring-like members for spacing saidelement from said top wall and from said bottom wall.
 6. The inventiondefined in claim 5 wherein said electromechanical transducing elementincludes a polarizable ferroelectric ceramic material.
 7. The inventiondefined in claim 6 wherein said ceramic material is selected from thegroup consisting of barium titanate, lead titanate-lead zirconate, andsodium potassium niobate.
 8. The invention defined in claim 7 whereinsaid spring-like member is a metal foil.
 9. An electroacoustictransducer comprising an electromechanical transducing element, ahousing defining an internal chamber having a top wall and a bottom wallfor containing said element, and at least one annular spring-like memberfor spacing said element from one of said walls, said member comprisinga plurality of interconnected generally circular leaves each having athickness and an annular width determined by the size of a central holeformed therein, said width being large in relation to said thickness.10. In an electroacoustic transducer including a transducing element anda housing for containing said element, means for supporting said elementin said housing comprising at least one annular spring-like memberhaving a plurality of interconnected leaves each having an annular widthrelatively large in relation to its thickness.
 11. An electroacoustictransducer comprising an electromechanical transducing element, ahousing defining an internal chamber having a top wall and a bottom wallfor containing said element, and means for supporting said elemeNtwithin said chamber, said supporting means including at least oneaccordion pleated member comprising a plurality of interconnectedgenerally annular leaves each having an annular width sufficiently greatto provide a measurable amount of acoustic damping.